White light emitting diodes (LEDs) have attracted attention as highly efficient and highly reliable white lighting light sources, and thus some white LEDs are already provided for use as small micro-power light sources. There are various methods of realizing white LEDs. However, the most common and currently used method includes molding a blue LED element using a resin as a matrix together with a yellow phosphor. However, the resin may easily deteriorate since blue light has strong energy. Accordingly, since the resin is discolored when a white LED is used for a long time, a color tone of light emitted from the white LED having such a structure changes. Also, since heat is not easily dissipated from the element since the element is molded by the resin, the temperature may easily rise. The color of emitted light may shift toward yellow due to such an elevated temperature.
To solve these problems, phosphor plates in which a ceramic sintered body is used as a matrix material of a phosphor have been applied. However, when a ceramic sintered body is used, transmittance of the ceramic sintered body may be lowered by 30% to 40%, compared to conventional resin matrices. In this case, a large amount of light from a blue LED light source is not incident due to low transmittance of the ceramic sintered body, which makes it impossible to sufficiently excite the phosphor. Also, since the light from the blue light source is not sufficiently incident on the phosphor plate (i.e., an optical wavelength conversion portion), a decrease in light flux may be caused due to a decrease in concentration of the phosphor added to emit light with desired color coordinates.
Further, an integrally formed phosphor plate is subjected to surface polishing and processing in order to mount it on a light source element. In this case, light flux extraction may be degraded due to an increase in reflectance of light on a flat surface of the phosphor plate obtained through the surface polishing.